The best way to protect gardenias from disease is by caring for them properly. Fungal diseases thrive on moisture. Plant gardenias in loose, well-drained organic soil. Do not over-water them and do not mist the foliage; gardenias are temperamental. Bud drop that plagues gardenias is not caused by disease, but rather by heat, cold spells, lack of sunlight, under-watering and over-watering.
The fungal disease Phomopsis gardeniae causes a canker to grow on the swollen main stem at or beneath the soil. The bark becomes corky and long cracks form by the canker. The stem is bright yellow above the canker, and the normal greenish-white beneath it. A yellowish substance may develop on the surface if the humidity is high. The disease stunts plants and they die slowly.
Destroy diseased plants and sterilize your pruning tools. No fungicides are available to treat cankers that strike gardenias
Bacterial Leaf Spot
The bacterias Pseudomonas gardinae and Xanthomonas campestris cause small round spots to develop on young leaves. The spots enlarge, and their centers change from pale yellow to a reddish-brown surrounded by a yellow halo. The edges of the lesions appear water-soaked and thick. The small spots may come together to form irregularly-shaped, large spots. The infected plant may lose its leaves.
Water at the roots, not overhead. Do not propagate infected plants. There are no chemicals that will control bacterial leaf spots. To prevent it from spreading, horticulturalists at the University of California, Davis, recommend fungicides containing copper hydroxide.
Rhizoctonia Leaf Spot
Rhizoctonia leaf spot is caused by the fungus Rhizoctonia spp. Tan to brown circular spots up to ¼ inch wide develop on the leaves. The spots begin on mature leaves and spread upward if the plants are watered too much, or if overcrowding causes air to circulate poorly.
Do not wet foliage when watering, and destroy diseased leaves. Only propagate plants that are disease-free. Spray a fungicide that contains chlorothalonil on the leaves. Follow the instructions on the label.
The fungi Cercospora spp. and Phyllosticta spp. cause spots on gardenia leaves all year along. These small, dark brown dead areas are surrounded by a yellow halo. The disease can cause leaves to drop prematurely. Spray regularly with a copper-based fungicide.
Sooty mold is caused by the fungus Capnodium spp. and appears as a thin black layer of fungus on top of the leaves. This fungus grows on sugary substances that white flies extrude. Most insecticides will only work if they come into direct contact with white flies.
Spray white flies thoroughly with neem oil or other insecticidal oil. Introduce natural predators including big-eyed bugs, minute pirate bugs, lacewings and lady bugs.
Powdery mildew caused by the fungus Erysiphe polygoni causes powdery white spots to form on leaves. Horticulturalists at the University of California, Davis, recommend fungicides containing sulfur or potassium bicarbonate or horticultural oils like jojoba oil and neem oil.